The present invention is related to a retrotelecentric lens in which lateral aberration is highly corrected and back focus is longer than focal distance.
An optical system of telecentric type must have a chief ray 3, which is incident upon an image plane 1, and is parallel with an optical axis 2 and for that end a diaphragm 4 must be provided at a position with a range of approximately .+-. 0.3f from the object side focal point 5. And the rear lens diameter of the lens is determined by the back focus, F-number and the amount of circumferential light and the larger the latter elements, the more the rear lens diameter increases. Therefore, especially in a so-called retro-telecentric type lens, to which a retro-focus type lens system having a back focus longer than the focal distance is applied, it is unavoidable that the rear lens diameter becomes excessively large. Thus, as a result of the lens diameter becoming large, a high degree of correction of aberration becomes difficult, and lateral aberration frequently takes place. FIG. 2 shows an example of the aberration of such conventionally known retrotelecentric lens as mentioned above, and as shown in this drawing, in a small angle that is at a central part of image plane many times inward, coma takes place, while in a large angle, that is at circumferential part of the image plane outward, coma takes place many times.